<!DOCTYPE html>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width">
<title>Dynamic sticky position change doesn't break inner sticky positioned items</title>
<link rel="help" href="https://drafts.csswg.org/css-position-3/#sticky-position">
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1908242">
<link rel="author" title="Emilio Cobos" href="mailto:emilio@crisal.io">
<link rel="author" title="Mozilla" href="https://mozilla.org">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
  body {
    margin: 0;
    display: flex;
  }

  #content {
    border: 10px dashed gray;
    height: 600vh;
  }

  #sidebar {
    align-self: start;
    background-color: white;
    border: 10px dashed gray;
    top: 0;
  }

  #sidebar-header {
    position: sticky;
    top: 0;
    background-color: rgba(255, 0, 0, .5);
  }

  #sidebar-content {
    padding: 5px;
    height: 200vh;
  }
</style>
<div id=content>
  CONTENT
</div>
<div id=sidebar>
  <div id=sidebar-header>
    SIDEBAR TOP STICKY
  </div>
  <div id=sidebar-content>
    SIDEBAR CONTENT
  </div>
</div>
<script>
let sidebar = document.getElementById("sidebar");
let sidebarHeader = document.getElementById("sidebar-header");
test(function() {
  // Make the header and sidebar stick.
  window.scrollTo(0, 100);
  assert_less_than(sidebar.getBoundingClientRect().top, 0, "Sidebar should not be stuck (yet)");
  assert_equals(sidebarHeader.getBoundingClientRect().top, 0, "Sidebar header should be stuck");
  sidebar.style.position = "sticky";
  assert_equals(sidebar.getBoundingClientRect().top, 0, "Sidebar should be stuck now");
  assert_equals(sidebarHeader.getBoundingClientRect().top, 10, "Sidebar header should be stuck under sidebar border");
});
</script>
